1. Field of the Invention
This invention relates in general to magnetic tunnel junction transducers for reading information signals from a magnetic medium and, in particular, to a dual hybrid magnetic tunnel junction/giant magnetoresistive sensor and to magnetic storage systems which incorporate such sensors.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a nonmagnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier layer 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Dual SV or MTJ sensors can provide increased magnetoresistive response to a signal field due to the additive response of the dual sensors. IBM""s U.S. Pat. No. 5,287,238 granted to Baumgart et al. discloses a dual SV sensor. FIG. 3 shows a dual spin valve sensor 300 wherein the spin valve structure is doubled symmetrically with respect a ferromagnetic free layer 308. The structure of the dual spin valve sensor is AFM1/Pinned1/Spacer1/Free/Spacer2/Pinned2/AFM2 providing ferromagnetic first and second pinned layers 304 and 312 separated by nonmagnetic first and second spacer layers 306 and 310, respectively, from a ferromagnetic free layer 308 which allows utilization of the conduction electrons scattered in both directions from the intermediate free layer 308. The directions of magnetization 305 and 313 (tails of arrows pointing into the plane of the paper) of the first and second pinned layers 304 and 312 are fixed parallel to each other by adjacent first and second antiferromagnetic layers 302 and 314, respectively. The direction of magnetization 309 of the free layer 308 is set at an angle of about 90xc2x0 with respect to the magnetizations of the two pinned layers and is allowed to rotate freely in response to an applied magnetic field.
Dual MTJ sensors, having insulating tunnel barrier layers in place of the conducting spacer layers of the dual SV sensor structure of FIG. 3, are of interest for their potential for high tunnel magnetoresistance response to an applied magnetic field. However, a ferromagnetic coupling field between the free layer and the pinned layers across the Al2O3 tunnel junction layers is usually very large ( greater than 20 Oe) because the tunnel junction layers are very thin. The coupling field from the two junctions add at the free layer making it difficult to achieve a proper bias angle for the free layer.
Therefore, there is a need for a dual MTJ sensor that provides the advantages of improved magnetoresitive coefficient without the problems associated with ferromagnetic coupling between the free layer and the pinned layers.
It is an object of the present invention to disclose a dual hybrid magnetoresistive sensor having a magnetic tunnel junction (MTJ) structure and a giant magnetoresistive (GMR) structure operating in the current perpendicular to the plane (CPP) mode.
It is another object of the present invention to disclose a dual hybrid MTJ/GMR sensor having a structure which reduces or essentially eliminates the net ferromagnetic coupling field between the free layer and the pinned layers.
It is a further object of the present invention to disclose a dual hybrid MTJ/GMR sensor having a structure that results in an improved bias point for the free layer.
In accordance with the principles of the present invention, there is disclosed a dual hybrid MTJ/GMR sensor comprising an MTJ stack separated from a GMR stack by a ferromagnetic free layer. Electrodes for providing sense current to the hybrid MTJ/GMR sensor are provided by a first shield and a second shield. Sense current flow is perpendicular to the plane (CPP) mode in both the MTJ stack and the GMR stack of the sensor.
The MTJ stack has an antiparallel (AP)-pinned layer separated from the free layer by an electrically insulating tunnel barrier layer. A first antiferromagnetic (AFM) layer adjacent to the first AP-pinned layer provides an exchange field to fix (pin) the magnetization direction of the first AP-pinned layer perpendicular to the ABS. The GMR stack has a second AP-pinned layer separated from the free layer by an electrically conductive spacer layer. A second AFM layer adjacent to the second AP-pinned layer provides an exchange field to pin the magnetization direction of the second AP-pinned layer perpendicular to the ABS.
The first AP-pinned layer comprises a first ferromagnetic (FM) layer adjacent to the first AFM layer, a second FM layer adjacent to the tunnel barrier layer and an antiparallel coupling (APC) layer sandwiched between the first and second FM layers. The second AP-pinned layer comprises a third FM layer adjacent to the spacer layer, a fourth FM layer and an APC layer sandwiched between the third and fourth FM layers. The second AFM layer is disposed adjacent to the fourth FM layer.
The first and second AFM layers have their magnetizations set in the same direction which results in the magnetization directions of the second and third FM layers adjacent to the tunnel junction layer and the spacer layer, respectively, to be parallel. Since the magnetoresistive responses of the MTJ stack and the GMR stack are functions of the relative orientations of the magnetizations of the second and third FM layers, respectively, with respect to the magnetization of the free layer, having the second and third FM layers pinned parallel to each other results in an additive response to a signal field of the MTJ and GMR stacks in the dual hybrid sensor.
An advantage of having the first and second AFM layers set in the same direction is that the same antiferromagnetic material may be used to form both layers and both layers can be set in the same process during fabrication.
A further advantage of the dual hybrid MTJ/GMR sensor is obtained by choosing the thickness of the spacer layer to provide negative ferromagnetic coupling between the third FM layer and the free layer across the spacer layer of the GMR stack. This negative coupling opposes the positive coupling between the second FM layer and the free layer across the tunnel barrier layer of the MTJ stack. It is known to the art that at low thicknesses of the electrically conductive spacer layer the ferromagnetic coupling increases and oscillates between positive and negative values. By choosing a thickness for which the ferromagnetic coupling field is negative, the net ferromagnetic coupling field at the free layer can be reduced to a small value resulting in an improved bias point for the free layer of the dual hybrid MTJ/GMR sensor.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.